Voltage-Sourced Converters in Power Systems

2

DC/AC Half-Bridge Converter

2.1 INTRODUCTION

This chapter investigates the DC/AC half-bridge converter as a building block for the multiphase, in particular three-phase, DC/AC voltage-sourced converter (VSC). In Chapter 5, based on the insight obtained through this chapter, we extend the model of the half-bridge converter to study the three-phase DC/AC VSC. This chapter presents the steady-state and dynamic models of the half-bridge converter to the extent that we will be able to directly exploit the results for the multiphase DC/AC VSC.

2.2 CONVERTER STRUCTURE

Figure 2.1 shows a schematic diagram of a DC/AC half-bridge converter [16].¹ The half-bridge converter is composed of two switch cells. The upper and lower switch cells are numbered 1 and 4, respectively. Each switch cell is realized by antiparallel connection of a fully controllable unidirectional switch and a diode. As discussed in Section 1.4.1.3, such a switch cell is also known as a reverse-conducting switch and is commercially available as IGBT or IGCT.² For ease of reference, throughout this chapter we refer to the fully controllable switch as the transistor, for which we have adopted the circuit symbol of the conventional bipolar junction transistor. Thus, the upper switch cell consists of the transistor Q₁ and the diode D₁. Similarly, the lower switch cell is composed of the transistor Q₄ and the diode D₄. As Figure 2.1 shows, in each transistor the positive current is defined as the current flowing from the collector ...